ADHD is the most prevalent mental health disorder of childhood. The majority of children with ADHD receive their care in primary care settings. While the American Academy of Pediatrics (AAP) issued evidence-based guidelines and recommendations for pediatricians, most pediatricians have difficulty adhering to these guidelines. Given observed deficiencies in evidence-based ADHD care and the likely effects on child outcomes, the development and testing of interventions aimed at improving ADHD care in primary care settings is necessary. Cincinnati Children's Hospital Medical Center has developed a model intervention, termed the ADHD Collaborative, to comprehensively address this issue. The ADHD Collaborative intervention model includes academic detailing, quality improvement methods, and innovative tools (e.g., web portal) designed to promote and support the systematic use of the AAP guidelines. This intervention model has been used to train over 200 physicians at 55 practices in the Greater Cincinnati area. The intervention appears to produce 2- to 4-fold increases in the use of evidence-based ADHD-related practice behaviors in participating physicians. To date, the intervention has been implemented as a quality improvement project with few experimental controls. The primary goal of the proposed study is to conduct an experimentally-controlled cluster randomized trial of the ADHD Collaborative intervention. Thirty-six pediatric practices will be randomly assigned to receive the ADHD Collaborative intervention or to provide usual care. Approximately 108 physicians and 648 of their ADHD patients will be included in the study. Chart reviews, parental interviews, and parent and teacher rating scales will be collected. Between- and within-group hierarchical linear modeling analyses will examine whether the intervention produces significant improvements in pediatrician practice behaviors, patient satisfaction with ADHD care, and child outcomes over and above typical ADHD care. Also, the relative cost effectiveness of the ADHD Collaborative intervention over typical care will be established by computing incremental cost-effectiveness ratios using cost and effect size estimates.